The networking of control units, sensor systems and actuator systems with the aid of a communications system and a communication connection, for instance in the form of a bus system, has increased drastically in recent years in the manufacture of modern motor vehicles or also in machine construction, especially in the field of machine tools, and in the automation field as well. Synergistic effects can be achieved by distributing functions to a plurality of control units. This is referred to as distributed systems.
The communication between various users of such a data transmission system is increasingly being implemented via a bus system. The communication traffic on the bus system, access and receiving mechanisms, as well as error handling are regulated by a protocol. One known protocol is, for instance, the FlexRay protocol, which is currently based on the FlexRay protocol specification v2.1. FlexRay is a fast, deterministic and error-tolerant bus system, especially for use in motor vehicles. The FlexRay protocol operates according to the principle of time division multiple access (TDMA), in which the users or the messages to be transmitted are assigned fixed time slots during which they have exclusive access to the communications link. The time slots repeat at a fixed cycle, so that the instant at which a message is transmitted via the bus can be predicted exactly, and the bus access takes place deterministically.
To optimally utilize the bandwidth for the message transmission on the bus system, FlexRay subdivides the cycle into a static and a dynamic portion. The fixed time slots are in the static portion at the beginning of a bus cycle. In the dynamic part the time slots are assigned dynamically. Therein, the exclusive bus access is always provided for only a short time, for the duration of at least one so-called mini slot.
The time slot is lengthened by the necessary time only if a bus access takes place within a minislot. Consequently, bandwidth is used up only if it is actually needed. In the process, FlexRay communicates via one or two physically separate lines at a data rate of maximally 10 Mbit/sec in each case. Of course, it is also possible to operate FlexRay at lower data rates. The two channels correspond to the physical layer, in particular of the so-called OSI (open system interconnection) layer model. They are used chiefly for the redundant and therefore error-tolerant transmission of messages, but are also able to transmit different messages, whereby the data rate would then double. It is also possible that the signal transmitted via the transmission lines results from the difference of signals transmitted via the two lines. The physical layer is designed such that it allows an electrical but also an optical transmission of the signal(s) via the line(s) or a transmission in some other manner.
To realize synchronous functions and to optimize the bandwidth by small intervals between two messages, the users in the communications network require a common time base, which is referred to as global time. For the clock synchronization, synchronization messages are transmitted in the static portion of the cycle, and the local clock time of a user is corrected with the aid of a special algorithm according to the FlexRay specification in such a way that all local clocks run in synchronism with a global clock.
In the transmission of data or messages via such a bus system, pulses are distorted because high-to-low or low-to-high edges are delayed to different degrees on the transmission path. If the transmitted pulse is sampled repeatedly (for instance, n-times per bit) in the receiver using the sample clock (the so-called sampling rate) available there, then the position of the sampling point, i.e., the selection of precisely one of these n sampling values, decides whether the datum is sampled correctly or incorrectly. This is difficult especially when the sampling instant refers to an edge of the signal and also analyzes a plurality of binary data values (bits) of the transmitter relative thereto, over many periods of the sampling period. In addition to a pulse distortion, the clock frequency deviation between transmitter and receiver also has an effect in this context. It has become apparent that the rigid specification of the sampling instant without considering the asymmetrical delays on the different transmission paths leads to problems.
Because of the rigid selection of the sampling instant per bit (for instance, at n=8 sampling values per bit, to 5, in the middle of a bit), both the influence of the asymmetrical distortion and the frequency deviation as well as the additional time discretization by the sampling pose a problem and place high demands on the transmission channel. Increasing the edge steepness so as to reduce the asymmetrical delays would indeed be advantageous for the timing, but on the other hand would require technically more sophisticated and thus more expensive components and, in addition, could have an adverse effect on the EMC response of the data transmission system. However, depending on the pulse distortion, there is the risk of evaluating the wrong datum either on the one or the other bit boundary.
When realizing FlexRay data transmission systems, in particular in the case of complex systems that include a plurality of star couplers and passive networks, it has also been shown that the asymmetrical delay times that occur there are so great that they exceed a time budget specified by the FlexRay protocol. According to the FlexRay protocol, a sampling counter is synchronized, i.e., reset, in response to a falling BSS (byte start sequence) edge. Sampling occurs at a counter reading of 5. In an eight-fold oversampling as it is currently provided in FlexRay, three sampling cycles thus remain between the sampling instant (fifth sampling value) and the eighth sampling value, which, given a communications controller cycle of 80 MHz, thus correspond to 12.5 ns in each case and therefore to a time budget of 37.5 ns in total. This time budget is actually provided to compensate for asymmetrical delays due to the difference between the falling and rising edge steepness. However, as may be the case in complex network topologies, if the asymmetrical delay exceeds the provided time budget, then an incorrect value will be determined in a sampling at the fifth sampling cycle (counter reading of the cycle counter at 5), since the particular bit that should actually have been sampled was already available at an earlier instant due to the asymmetrical delay and is no longer present because of the early edge change. An analogous treatment applies to an asymmetrical delay to retard. In that case, a time budget of four sampling cycles is available, which corresponds to 50 ns. Exceeding the time budget in the advance or retard direction results in decoding errors, which means that incorrect data are received.
These decoding errors may actually be detected by suitable error detection algorithms, so that renewed transmission of the bit or the entire data frame may be initiated. A cyclic redundancy check (CRC), for example, may be used as error detection algorithm. However, if the error detection algorithm responds too frequently, there is the disadvantage of reduced availability of the data transmission system.
In summary, it may be said that the FlexRay protocol sets down stipulations that the physical layer, at least with complex network topologies, is unable to meet.